Precise control of the various functions of a crane, e.g. swing, boom angle and rope drums, is critical. In many cranes control of these functions is by means of hydraulically actuated clutches, and consequently the engagement of these clutches must be precisely controlled. Modulation of the flow is not as effective a means for control of clutch engagement as is the modulation of the pressure to these clutches, because the force of clutch engagement is directly related to the pressure in the clutch. A variable pressure valve arranged to be mechanically shifted by a manual control lever is relatively simple to implement and is very effective in delivering hydraulic fluid under variable pressure to an associated clutch. However, the use of a mechanical connection between the control lever and the valve is often difficult and invariably includes pivot connections which are subject to wear, require frequent maintenance, such as lubrication, and often need periodic adjustment and/or replacement of certain parts. In addition, a mechanical control system is limited to providing control capability from a single location. In certain crane installations, e.g. offshore platforms, it is often desirable for an operator to maintain visual contact with the load to be picked or placed.
In the case of an offshore platform, for example, the platform supporting the crane is elevated a considerable distance above the water level. A ship containing the load to be picked by the crane moves vertically in response to wave motion on the surface of the water. In order to avoid damage to the ship and/or the load, it is important that picking of the load be initiated at or near the peak of the ship's vertical motion because the rate at which the ship moves upward under the influence of the wave is often faster than the rate at which the crane can raise the load. It is, therefore, advantageous for the crane operator to observe the relation of the ship and the waves, which relationship is obscured from the operator's cab by the deck of the platform.
The present invention permits pressure modulation of the control clutches in a crane selectively from more than one location, provides a simplified control system which minimizes pivot connections and the concomitant maintenance and repair problems, and which minimizes or is devoid of the aforementioned problems, disadvantages or limitations.